Small-and-medium-sized hydroelectric power plants are generally run-off-river-type plants. In such a plant, an output varies depending on a flow rate of a river. If the flow rate of the river drops considerably, a strong swirling flow may be produced and cavitation may occur at a runner outlet of a water turbine. As a result, large vibrations and loud noise may occur in a tube of the plant, and stable operation will be difficult. Therefore, in general, a minimum flow rate which allows the power plant to operate is determined in advance, and when the flow rate drops below the minimum, operation of the power plant facility is stopped. As a technique to suppress vibrations which may occur during operation under partial load of a water turbine, an improved arrangement of fins attached to a draft tube of a water turbine is known.
When the flow rate of the river is less than the minimum determined in advance, the river water at that time is discharged without being used for power generation. Thus, energy is wastefully released. However, even when the flow rate of the river is low, if the rotation speed of the runner is reduced, the swirling can be weakened and the vibrations and noise can be suppressed. Accordingly, stable operation can be conducted.
However, if the power generator is directly connected to the power system, the rotation speed will be kept constant during operation. Thus, the rotation speed cannot be changed.
Under these circumstances, it is desired to provide a variable-speed operation control apparatus and a hydroelectric power generation system, in which even when the flow rate of a river or the like is less than a fixed level, a stable operation can be conducted by reducing the rotation speed.